BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 21, pp. 731-736 December 31, 1910 



TABLES FOR THE DETERMmATION OF CRYSTAL CLASSES^ 

 {Bead before the Cor dill eran Section of the Society March 25, 1910) 



BY W. S. TANGIER SMITH 



CONTENTS 



Page 



Introduction 731 



Table No. 1 732 



A. With center of symmetry 732 



B. Without center of symmetry 733 



Table No. 2 734 



A. With no planes of symmetry 734 



B. With one or more planes of symmetry 735 



Introduction 



One of the difficulties met by beginners in crystallography is the deter- 

 mination of the class to which a given crystal belongs. With a view to 

 lessening this difficulty, the writer about two years ago prepared separate 

 keys for the determination of the different classes belonging to each 

 crystal system, and later a more comprehensive single key, including all 

 the crystal classes. As this key has proved very satisfactory in use in the 

 writer's classes, it is presented here in the hope that it may be of service 

 to others. Various forms of the key have been worked out from time to 

 time in the effort to find the simplest and most satisfactory arrangement, 

 the two tables given here being those which have seemed the best in 

 practice. 



The first of these tables makes use of a center of symmetry as a basis 

 for its main divisions, adopting the simpler of the two current definitions 

 of that term; that is, the only condition assumed as necessary is the 

 occurrence, for every plane on the crystal, of a corresponding, like, 

 parallel plane on the opposite side of the center. 



The other table has been so arranged as to require no reference to a 

 center of symmetry; and at the same time the forms have been grouped 

 in accordance with the recently proposed classification of Swartz.^ 



1 Manuscript received by the Secretary of the Society June 27, 1910. 



2 Charles K. Swartz : Proposed classification of crystals based on the recognition of 

 seven fundamental types of symmetry. Bull. Geol. See. Am., vol. 20, 1908, pp. 369-391. 



(731) 



